a) Field of the Invention
The present invention relates to an exposure technique, and more particularly to an exposure technique for focussing a fine pattern of an LSI or the like.
b) Description of the Related Art
As the wavelength of an exposure beam becomes comparable to the degree of the resolution of a pattern to be focussed, the diffraction and interference of the exposure beam become an important issue. Under such a condition, if a mask pattern having the same shape of a desired device pattern is used, the desired device pattern will not be transferred.
The device pattern of a semiconductor integrated circuit device or the like is becoming fine more and more. In light exposure using a light beam, the diffraction and interference of light become a main issue as the device pattern becomes fine.
In the exposure using an optical system, the resolution is a function of a numerical aperture NA and a wavelength. In order to improve the resolution, it is desired to make the numerical aperture large and the wavelength short. There is a limit in increasing the numerical aperture because of the depth of focus and the design of the optical system. For shortening the wavelength, the bright line of a mercury lamp is partly replaced by an excimer laser beam, and such a short wavelength is now considered ultimate because of the restriction of optical material to be used.
As the pattern size becomes in the order of a light wavelength, the size of a fine pattern to be transferred is limited by the light beams diffracted to the outside of the pattern.
A phase shift mask has drawn attention, which positively uses the light interference in order to improve the resolution of photolithography.
A phase shifter relies upon the technique of improving the resolution by using light beams of different phases. For example, a phase shifter is formed around the main hole corresponding to a hole pattern to be transferred. The phase shifter compensates for light beams passed through and diffracted by the main hole. The phases of light beams from the main hole and phase shifter are different, e.g., opposite phases, reducing the light intensity at the projection plane. A high resolution which was conventionally impossible can now be attained by using the phase shifter.
With the phase shifter, however, various problems occur when a pattern to be exposed becomes very fine.
Specifically, if a phase shifter is formed around each unit pattern of a repetitive pattern such as a pattern used for an ULSI or the like, sub peaks may be generated between transferred unit patterns. Generation of a sub peak may result from the added intensities of light beams from the main hole and phase shifter. In such a case, the width of the phase shifter is reduced to prevent the generation of sub peaks.
If the width of a phase shifter best designed is reduced to prevent the generation of sub peaks, the intrinsic effects of the phase shifter are suppressed and degraded, making it difficult to obtain a sufficient contrast.
A depth of focus or field is desired more or less in order to transfer a pattern to the surface of a structural subject such as a semiconductor device. In some cases, a sufficient depth of focus may not be obtained even if the resolution on the focal plane is improved by the phase shifter.
If a phase shifter is used for a mask, a light source is disposed along the optical axis of a condenser lens. By using a point light source on the optical axis, the occupation factor or coherence factor .sigma. of the light source by the condenser lens can be made small. Under this condition, the pattern of the mask is transferred to a photoresist. A phase shifter includes a Levenson type, Cr-less type, edge emphasized type, and other type shifters.
Illuminance lowers when such a point light source is used with a small coherence factor .sigma.. It is difficult to obtain a sharp pattern because of a lower radiation energy. If the coherence factor .sigma. is raised to solve this problem, the interference ability lowers, the intrinsic effects of the phase shifter cannot be obtained, and the depth of focus becomes poor.
In addition to the phase shifter, there is known a mask design method capable of obtaining an optimum light intensity distribution on the focus plane, by designing the pattern of the mask independently from the pattern to be focussed. This method may be used singularly or in combination with the phase shifter.
This method relies mainly on empirical knowledge or a rule of thumb. Even if a device pattern is designed with long time and much labor, the result may often be trapped at a local solution.
The light intensity distribution on the image plane depends on the performance of all optical components throughout the optical path from a light source to the image plane. The distribution of light rays illuminating a mask greatly influences the focussing performance.